The present invention relates generally to a mechanical blade-type chain tensioner for use in an automotive timing drive and, more particularly, to a plastic blade (shoe) member having improved structural rigidity at the spring reaction regions/features of the blade to in order to minimize deflections at these regions under maximum spring loading, thereby affording a greater level of dynamic stability for the blade and spring assembly during high speed engine operation.
FIGS. 1 & 2 show an exemplary known blade-type tensioner T′ that is particularly suited for use in confined spaces. The tensioner T′ comprises a blade assembly BAS′ operatively connected to a support bracket K. The blade assembly BAS′ comprises a metal leaf spring S that is mechanically interlocked with a polymeric (plastic) shoe or blade B′ for applying a tensioning force to the slack strand segment 17 of a chain 15. The spring S is typically formed as a leaf spring from a generally rectangular one-piece strip of spring steel that is formed to have an arched shape. The spring S can alternatively comprise a nested stack of leaf springs. The bracket K includes a pivot pin (PIN) that projects outwardly from a main wall MW, and a first or pivot end B1′ of the blade assembly BAS′ includes a pivot barrel BL′ in which is defined a pivot bore PB′ that receives the pivot pin PIN for reciprocating clockwise/counter-clockwise pivoting or angular movement of the blade assembly BAS′ about the pin PIN. The bracket K also includes a ramp R that also projects from its main wall MW, and an opposite, second or free end B2′ of the blade assembly BAS′ is supported on the ramp R for reciprocal sliding movement. The blade B′ also includes a central segment or portion B3′ that extends between and connects the first and second ends B1′,B2′, and an outside surface OS′ of the central portion B3′ provides a chain contact surface adapted to be slidably engaged by the associated chain 15 being tensioned. The bracket K thus maintains the blade assembly BAS′ in its proper position with respect to the plane of the chain path while permitting sliding reciprocal translational motion of the second, free end B2′ on the ramp R as indicated by the arrow “TRANS” along with the related rotational movement of the blade assembly BAS′ at the pivot end B1′ as indicated by the arrow labeled “ROTATE” in response to changes in the tension and position of the slack strand 17 of the chain 15 and corresponding oscillatory movement of the slack strand 17 and blade central portion B3′ as indicated by the arrow “AMPL.”
FIG. 2 illustrates the tensioner T′ secured to an associated engine block EB as part of the timing drive system and is shown in contact with a slack strand of a new (unworn) timing chain 15. The preferred orientation of the outside surface OS′ of the blade central portion B3′ with respect to the slack strand segment 17 of the new chain 15 is for the chain contact length to be substantially centered at a midpoint of the central blade segment B3′ which also defines and is aligned with the midpoint SMP of the spring S (see also FIG. 3). Referring to FIG. 2A, the blade assembly BAS′ is shown at its full working travel position in contact with a max elongated (worn) chain 15. This full working travel operative position for the blade assembly BAS′, shown in solid lines, is overlaid with the blade assembly (in phantom lines) at its initial operative position with a new chain 15.
With reference also to FIG. 3, the first and second ends B1′,B2′ of the blade define respective first and second spring-receiving slots SL1′,SL2′ for respectively receiving and retaining first and second opposite ends S1,S2 of the spring S. The blade central portion B3′ includes a lower or inner surface IS′ that is defined by the underside of the central portion B3′ that is opposite the outer surface OS′. The inner surface IS′ is contacted by an arched central portion S3 of the spring S. As such, the first and second spring-receiving slots SL1′,SL2′ and the inner surface IS′ of the blade central portion B3′ define a spring-receiving slot or region that opens through the opposite front and rear faces FF′,RF′ of the blade B′. The axial retention of the spring S to the blade B′ is achieved with the walls W1′,W2′ at the front face FF′ side of the blade and the retaining tabs T1′,T2′ at the rear face RF′ side, but this arrangement can be reversed. In particular, the first end S1 of the spring S is retained in the first slot SL1′ between a first side wall W1′ and a first installation tab T1′, and the second end S2 of the spring S is retained in the slot SL2′ between a second side wall W2′ and a second installation tab T2′. The wall and retaining tab configuration and method of spring installation for the blade assembly BAS′ is disclosed in Young, U.S. Pat. No. 9,206,886, the entire disclosure of which is hereby expressly incorporated into the present specification. The walls W1′,W2′ and retaining tabs T1′,T2′ are shown in FIG. 3, but are omitted from some of the other figures for clarity in illustrating the underlying features.
With continuing reference to FIG. 3, the blade assembly BAS′ is shown at its initial operative position for a new (shortest) chain 15. A first end wall E1′ of the blade B′ extends transversely between and connects the first lower wall LW1′ to the blade central portion B3′ and closes the first slot SL1′ at the first end B1′ of the blade B′. Similarly, a second end wall E2′ of the blade B′ extends transversely between and connects the second lower wall LW2′ to the blade central portion B3′ and closes the second slot SL2′ at the second end B2′ of the blade B′. The opposite first and second ends S1,S2 of the spring S are respectively located in the first and second slots SL1′,SL2′, and the first and second spring ends S1,S2 respectively include first and second linear edges SE1,SE2 (see also FIGS. 3E & 3F) that are seated in line contact with the respective lower slot wall surfaces LW1′,LW2′ at respective first and second contact positions or contact locations 10,12 as defined by the effective spring length LE. The arched central portion S3 of the spring contacts the underside or inner surface IS′ of the central portion B3′ of blade B′.
The purpose of a chain tensioner T′ for an automotive timing drive system is to provide a sufficient tensioning force to the slack chain strand 17 in order to properly control the transverse chain motion and the torsional vibrations resulting from the loading and torsional inputs such as the valve events at the camshaft and the firing pulses at the crankshaft. These firing engine dynamic inputs will generally cause the tensioner blade assembly to stroke dynamically—but in a controlled manner if the device is properly engineered for the engine. The tensioner must also have sufficient take-up capability as the chain wears and elongates in service in order to continue to properly control the chain strand at the elongated (worn) chain lengths.
The blade assembly BAS′ in FIG. 3A is shown at its initial operative position as it would be positioned when a slack strand chain segment 17 of a new chain is in contact with the outer surface OS′ of the central blade segment B3′ at the spring midpoint SMP (see also FIG. 2 for a corresponding view that also shows the chain strand 17). FIG. 3D shows the spring S by itself in the initial operative position corresponding to FIG. 3A, and it can be seen that the spring S defines a height h1 relative to the reference line of length LE connecting the first and second edges SE1,SE2 of the spring (i.e., the reference line connects the contact locations 10,12).
With continuing reference to FIG. 3A, a free body diagram of the blade assembly BAS′ at its initial operative position is shown and with the system in equilibrium, a summation of the force vectors acting on the blade assembly will equal zero. The blade assembly will exert a force SFMAX against the chain strand as a function of the deflected height of the tensioner spring S and the chain strand 17 will exert an equal and opposite force CFMAX against the blade outer surface OS′. The chain tensioning force for a known prior art tensioner at the new chain position is 20 lbs. The force vectors acting on the blade, shown with solid black fill, are the vertical chain force CFMAX acting against the outer blade surface OS′ at the blade midpoint, the bracket ramp force FR acting at a free end B2′ of the blade against the blade foot BF′ at the contact location 14 normal to the ramp surface R at an angle θ with respect to a reference line LREF2 oriented normal to the line of length LE that connects the opposite edges SE1,SE2 of the spring S (the θ reference line LREF2 is vertically oriented in the illustrated example), and the bracket pivot pin force FP acting at the pivot end of the blade against the inside diameter of the blade pivot bore PB′ through its center or axis of rotation P′ at contact location 18 and also at an angle θ relative to a reference line LREF1 also oriented normal to the line of length LE to cancel out the horizontal components of the force vectors FR,FP. The summation of the external force vectors acting on the blade equal zero, and the force vectors FR,FP are substantially equal. Similarly, the spring force vectors SFMAX,SFR,SFP, shown with crosshatch fill, act internal to the blade B′ and the summation of these force vectors also equals zero. The spring force vectors SFP and SFR act on the slot lower walls LW1′,LW2′ at the respective contact locations 10,12 where the first and second spring edges SE1,SE2 contact the lower walls LW1′,LW2′.
FIG. 3B is an enlarged partial view of the FIG. 3A blade at the free end ramp and FIG. 3C is an enlarged partial view of the FIG. 3A blade at the pivot end. As shown in FIG. 3B, the force vectors SFR and FR are offset with respect to each other by a distance or moment arm dR′ at the blade free end B2′ due to a corresponding offset between the contact locations 12 and 14. Similarly, as shown in FIG. 3C, the force vectors SFP and FP are offset with respect to each other by a distance or moment arm dP′ at the blade pivot end B1′ due to a corresponding offset between the contact locations 10 and 18. These moment arm offsets dP′,dR′, will produce or result in a repeated deflection of the lower wall surfaces LW1′,LW2′ of the first and second spring-receiving slot SL1′,SL2′ toward the surface IS′ of the blade central portion B3′ during engine operation, which is believed to have an adverse effect including relative motion between the spring edges SE1,SE2 and the respective lower walls LW1′,LW2′ which can result in abrasion of the lower walls LW1′,LW2′ and/or undesired longitudinal motion of the spring S toward and away from the first and second end walls E1′,E2′.
The camshaft and crankshaft torsional vibrations promote excessive chain drive dynamics at the higher engine speeds, particularly at 5,000 rpm and above, resulting in greater slack strand chain oscillatory motion along with an associated maximum blade assembly transverse motion (AMPL) for a given chain length. The amount of AMPL is a function of the chain length and AMPL increases with chain wear elongation.
The moment arm offsets dP′,dR′ are a major contributor to lower wall deflection and this deflection is largest at the higher tensioner loads during high speed engine operation. This deflection at high speeds is believed to result in a high frequency flexing of the lower wall surfaces LW1′,LW2′ as the blade and spring assembly motion AMPL cycles between the minimum operative tensioner loading position shown in FIG. 4 and the maximum operative tensioner loading position shown in FIG. 3A. It should be noted that the FIG. 4 position represents the max AMPL motion with a max worn (elongated) chain length but the detrimental flexing will still occur with a shorter worn chain. It is also important to note that the high frequency flexing results from the change in deflection or “delta” deflection between the two positional limits at high engine speed and it is believed to facilitate a partial unseating or floating of the springs ends SE1,SE2 relative to the lower wall surfaces LW1′,LW2′, thereby initiating relative longitudinal motion between the spring ends and the lower wall contact surfaces to cause the blade failure mode of end wall fracture as the spring slices its way through one or the other of the end walls E1′,E2′. This failure has been shown to occur at the blade end where there is excessive lower wall deflection during high speed operation.
Finite element analysis (FEA) and engine testing support the premise that the lower wall flexing during high speed engine operation will facilitate a cutting or slicing action by the spring ends SE1,SE2 at the end walls E1′,E2′ by initiating relative longitudinal motion between the spring ends SE1,SE2 and the lower wall surfaces LW1′,LW2′. This end wall cutting action occurs as the blade flattens out at the max loading blade position.
FIG. 3E shows that the spring S has variable heights h1,h2,h3 depending upon its operative state, with corresponding variations in the straight-line distance between its first and second edges SE1,SE2. FIG. 3F provides a bottom view of the spring S at height h1. The spring S has a free height h3 when unconstrained as in a free state. The height h1 corresponds to the height of the spring S for the initial operative position of the blade assembly BAS′ with a new chain 15 in which the spring defines the distance LE between its ends. The height h2 is equal to the height of the spring S when the blade assembly BAS′ is at its full working travel position in contact with a max elongated (worn) chain 15, at which condition the straight-line distance between the spring edges SE1,SE2 is reduced to LEFT which is less than LE. Testing has shown that when the blade assembly BAS′ is used in an engine with a worn chain, the blade assembly will oscillate between the FIG. 3A and FIG. 4 positions, corresponding to changes in the spring height from h1 to h2, and this magnitude of spring height change is referred to herein as the working travel WT of the spring S.
FIG. 4 shows that, as the chain 15 wears and the slack strand 17 lengthens to its maximum design length, the blade assembly BAS′ will move to its full travel operative position corresponding to the spring height h2 and the distance LEFT defined between the spring edges SE1,SE2. Because the pivot axis P′ about which the pivot end of the blade assembly BAS′ rotates is fixed, the blade assembly BAS′ must rotate about the pivot axis P′ slightly (clockwise in FIG. 4) as the chain lengthens and the spring height increases from h1 to h2. The contact location 14 of the blade foot BF′ on the ramp R shifts to contact location 14FT on the ramp R, as the blade assembly BAS′ rotates. To further illustrate the rotation of the blade assembly BAS′, the FIG. 3 (new chain) positions for the first and second spring contact locations 10,12 in the slots SL1′,SL2′ are also shown in FIG. 4 using broken leader lines. In FIG. 4, a free body diagram of the blade assembly BAS′ is shown at its full travel operative position and with the system in equilibrium, a summation of the force vectors acting on the blade assembly will equal zero. The blade assembly BAS′ will exert a force SFFT against the chain strand as a function of the deflected height of the tensioner spring S and the chain strand 17 will exert an equal and opposite force CFFT against the blade outer surface OS′. The chain tensioning force for a known prior art tensioner at this max length worn chain position is 10 lbs. The force vectors acting on the blade, shown with solid black fill, are the chain force CFFT acting against the outer blade surface OS′ at the blade midpoint, the bracket ramp force FRFT acting at a free end of the blade against the blade foot BF′ at the contact location 14FT normal to the ramp surface R at an angle β′ with respect to the reference line LREF2 oriented normal to the line of length LEFT that connects the opposite edges SE1,SE2 of the spring S, and the bracket pivot pin force FPFT acting at a pivot end of the blade against the inside diameter of the blade pivot bore PB′ through its pivot axis/center P′ at contact location 18FT and also at an angle β′ to cancel out the horizontal components of the force vectors FRFT,FPFT. The summation of the external force vectors acting on the blade equal zero, and the force vectors FRFT,FPFT are substantially equal. Similarly, the spring force vectors SFFT,SFRFT,SFPFT, shown with crosshatch fill, act internal to the blade B′ and the summation of these force vectors also equals zero. The spring force vectors SFPFT and SFRFT act on the slot lower walls LW1′,LW2′ at the respective first and second contact locations 10,12.
FIG. 4A is an enlarged partial view of the FIG. 4 blade at the free end ramp and FIG. 4B is an enlarged partial view of the FIG. 4 blade at the pivot end. As shown in FIG. 4A, the force vectors SFRFT and FRFT remain offset with respect to each other by a distance or moment arm dRFT′ at the blade free end due to a corresponding offset between the contact locations 12 and 14FT. Similarly, as shown in FIG. 4B, the force vectors SFPFT and FPFT are offset with respect to each other by a distance or moment arm dPFT′ at the blade pivot end due to a corresponding offset between the contact locations 10 and 18FT. These offsets dPFT′,dRFT′ are believed to produce or result in a repeated deflection of the lower wall surfaces LW1′,LW2′ of the first and second spring-receiving slot SL1′,SL2′ toward the inner surface IS′ during engine operation which is believed to have an adverse effect as described above for the offsets dP′,dR′, although the force vectors FRFT and FPFT each have a reduced magnitude relative to the respectively corresponding force vector FR,FP for the FIG. 3A initial operative position for a new chain 15.